Probe-based data storage has long been recognized as a possible basis for ultrahigh density data storage. In probe-based data storage devices, stored data is represented on a storage surface by the presence and absence of indentations, or “pits”, which are formed in the surface by a probe of the device. For example, in AFM (Atomic Force Microscope)-based storage devices, the probe is a nanometer-sharp tip mounted on the end of a microfabricated cantilever. This tip can be moved over the surface of a storage medium in the form of a polymer substrate. A mechanism is provided for heating the tip, and thus the polymer surface at the point of contact with the tip, allowing the tip to penetrate the surface to create a pit. Such a pit typically represents a bit of value “1”, a bit of value “0” being represented by the absence of a pit at a bit position on the storage surface. In a read-scan mode, the thermomechanical probe mechanism can be used to read-back data by detecting the deflection of the cantilever as the tip is moved over the pattern of bit indentations. AFM-based data storage is described in detail in IBM Journal of Research & Development, Volume 44, No. 3, May 2000, pp323-340, “The ‘Millipede’—More Than One Thousand Tips for Future AFM Data Storage”, Vettiger et al., and the references cited therein. As described in this document, while basic read/write operations can be implemented using a single cantilever probe, in practice an integrated array of individually-addressable cantilevers is employed in order to increase data rates.
A basic write operation in these probe-based storage devices thus involves creation of a pit in the storage surface by a micromechanical probe mechanism. As the pit is created, material is displaced by the probe and deposited in the area around the indentation. In the AFM-based device discussed above for example, melted polymer is deposited in the area around a pit creating “donut walls”, or “rings”, which have a higher topological profile than the polymer background. If a pit is formed sufficiently near to an existing pit, the displaced material can partly fill the existing pit, effectively changing a “1” to a “0” and thus erasing the “1”. The physical mechanism exploited for probe-based data storage thus imposes a limit on the smallest distance between pits created during a data write operation. This minimum distance Tmin is the smallest distance between adjacent pits created during a write operation for which writing the second pit will not erase the first pit due to displacement of material. For basic data write operations this limit Tmin translates into a minimum allowable distance between stored data bits.
When overwriting of old data with new data is contemplated in probe-based storage devices, certain difficulties arise as a consequence of the mechanism employed for data storage. For example, since a zero corresponds to “no pit” at a bit position, writing a zero at a bit position corresponds to no action. Hence, “writing” a zero over a previously-written “1” at a bit position will leave the “1” in tact, rendering the newly-written data incorrect. Because of such problems, prior to our copending European Patent Application No. 02010648.0 filed 13 May 2002, probe-based data storage relied on the writing of data on “clean” storage surfaces, i.e. surfaces on which data has not previously been written or from which old data has been erased. For example, in the case of the Millipede device discussed earlier, old data can be erased by heating the storage surface to cause melting and “reflow” of the polymer. However, erasing of old data is clearly a power and time intensive process. In our European patent application referenced above, a system is disclosed which allows direct overwriting of old data with new data in probe-based data storage devices. This system effectively involves a two-stage coding process. In the first stage, input data is coded to prevent occurrence of two consecutive bits of a given value “x” in the coded input data. If x=1 for example, then the coding ensures that successive “1's” in the coded input data are separated by at least “d” zeros, where d is a predetermined number≧1. The effect of this coding is to ensure that successive pits to be left in the storage surface after a write operation are always separated by at least one “no-pit”. This coding therefore allows the stored bit density to be increased over the uncoded case where the minimum bit spacing is constrained to Tmin as explained above. In the second coding stage, an algorithm is employed to generate an overwrite bit sequence from the coded input bit sequence. In simple terms, the algorithms employed here are based on the premise that writing a pit can erase an existing, neighboring pit on either side of the new pit due to displacement of material as discussed above. When the resulting overwrite bit sequence is written on the storage surface, the effect is to record either the original coded input bit sequence, or the complement of this bit sequence, depending on particular operating parameters. This result is independent of the bit values of the old data which is overwritten, and hence erasing of old data is not required.
The power expenditure in any data write operation in probe-based storage devices is directly related to the number of pits formed in the storage surface. From the principles underlying the overwrite system outlined above, it follows that writing an overwrite sequence generated from a given coded input bit sequence necessarily involves writing more pits than would writing the coded input bit sequence directly (though the latter operation would not of course allow overwriting of old data). The overwrite operation thus requires more power for given input data than a conventional write operation on a clean storage surface. Power consumption, always an important consideration, is thus a more prominent factor where direct-overwrite capability is provided. The present invention enables direct overwriting to be achieved with reduced power consumption compared to methods disclosed in our European patent application referenced above.